By means of such detectors, for example, the variation over time of the transmission of an eluting liquid is determined in order to obtain a chromatogram. The eluting liquid (below: sample) is led through the flow cell and irradiated with suitable light longitudinally or transversely with respect to the flow direction. In the case of absorption measurement, depending on the characteristics of the sample, the light is absorbed by the sample with different intensity at different wavelengths, so that only the non-absorbed proportion enters an optical conductor at the end of the flow cell and is fed to a suitable evaluation unit for further analysis.
The flow channel of the flow cell, through which the sample and the light are led, is usually formed as a thin tube. The light led along the tube is reflected on the tube inner side or—in the case of a transparent material of the tube wall—also on the outer side of the tube and thrown back into the interior of the tube. If, over a certain part of the distance between light inlet and light outlet, the light is propagated in the wall of the tube and not in the absorbing sample, systematic measurement errors occur, in particular in the case of highly absorbing samples. The wall thickness of the tube is preferably therefore chosen to be small, in particular in relation to the diameter of the fluid channel, but this makes the tube particularly sensitive to mechanical stress. In order to avoid any disruption to the reflection behavior on the outer tube wall, it is also necessary to prevent the contact of other substances with the outer side of the tube. Such a thin tube must be handled with great care in order to avoid damage during mounting and when mounted.